The synucleinopathies, including Dementia with Lewy bodies (DLB), Multiple system atrophy (MSA), and Parkinson's disease (PD), are a group of neurodegenerative disorders characterized by the accumulation of alpha-synuclein (a-syn), a small neural- specific protein that aberrantly aggregates into amyloid fibrils that comprise Lewy bodies, the characteristic pathological inclusions found in synucleinopathies. One of the critical issues in PD research, as well as across several neurodegenerative disease research areas, is how to target the specific toxic, pathogenic moiety for therapeutic development. The in vivo processes and cellular factors that control the formation of a-syn toxic forms are largely unknown. The recent discovery of parkinsonism and Lewy bodies in patients with Gaucher disease (GD), a rare lysosomal storage disorder characterized by mutations in the gene (GBA1) encoding glucocerebrosidase (GC)ase, suggests a link between lysosomal sphingolipid metabolism and a-syn aggregation. In our preliminary data we show that depletion of GCase results in endogenous a-syn accumulation and neurodegeneration in mouse and C. elegans models of GD. Compromised GCase activity in neurons led to accumulation of the GCase substrate glucosylceramide, diminished lysosomal function, and increased soluble a-syn oligomeric intermediates that were neurotoxic. Importantly, we found that a-syn accumulation has the ability to affect the lysosomal maturation and activity of normal GCase in neurons and human brain, suggesting that GlcCer accumulation also plays a role in sporadic PD and other synucleinopathies. The experiments proposed in this application will further test the hypothesis that alterations in GlcCer metabolism contribute to the pathogenesis of synucleinopathies. We will examine whether therapeutic targeting of mutated or normal glucocerebrosidase to lysosomes prevents or diminishes formation of toxic alpha- synuclein oligomers and breaks the vicious cycle of alpha-synuclein aggregation and toxicity. If successful, these studies will provide further validation for a role of lysosomal GCase in synucleinopathies and identify a specific molecular pathway for the development of new therapies for PD and related diseases characterized by accumulation of a-syn.